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        <td class="header">&nbsp; Ionospheric Correction<br>
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<h3>Ionospheric Correction Operator</h3>
<p class="MsoNormal">This operator uses three interferometric products (each of which with an unwrapped interferometric phase band)
    to estimate and correct for ionospheric phase screens. The estimation is done in two steps. First, an initial phase screen
    is computed. Then, a spatially-adaptive Gaussian filter is applied to reduce the noise level.
    The methodology adopted is based on [1].

<h4>Input<br>
</h4>
<p class="MsoNormal">
    The input to this operator should be three interferometric products, as read (in any order) by using the
    <b>ProductSet-Reader</b>. These input products can be created as follows:
<ul>
    <h5>TOPS</h5>
    <li>Coregister a pair of TOPS images using the usual workflow.
    </li>
    <li>Coregister a pair of <b>low</b> subbands of TOPS images. This can be achieved by calling the <b>BandPassFilter</b> operator
        right after calling <b>TOPSAR-Split</b>.
        Recommendation: reuse the overall azimuth/range shifts estimated by the <b>Enhanced-Spectral-Diversity</b> operator in the previous step.
    </li>
    <li>Analogously, coregister a pair of <b>high</b> subbands.
    </li>
    <li>Create interferograms (with coherence estimation enabled) for each coregistered dataset and deburst.
        Recommendation: use the <b>GoldsteinPhaseFiltering</b> and <b>Multilook</b> operators to filter the interferograms.
    </li>
    <li>Unwrap each interferometric phase band using SNAPHU. The name of the unwrapped phase band should contain the string
        <b>Unw_Phase_ifg</b> in order for the <b>IonosphericCorrection</b> operator to recognize it.
    </li>

    <h5>Stripmap</h5>
    <li>Coregister a pair of Stripmap images using the usual workflow.
    </li>
    <li>Coregister a pair of <b>low</b> subbands of images. This can be achieved by calling the <b>BandPassFilter</b> operator
        right after reading the original SLC.
    </li>
    <li>Analogously, coregister a pair of <b>high</b> subbands.
    </li>
    <li>Create interferograms (with coherence estimation enabled) for each coregistered dataset.
        Recommendation: use the <b>GoldsteinPhaseFiltering</b> and <b>Multilook</b> operators to filter the interferograms.
    </li>
    <li>Unwrap each interferometric phase band using SNAPHU. The name of the unwrapped phase band should contain the string
        <b>Unw_Phase_ifg</b> in order for the <b>IonosphericCorrection</b> operator to recognize it.
    </li>
</ul><br>

<h4>Output<br>
</h4>
<p class="MsoNormal">
    The output of this operator is a product with the following bands:
<ul>
    <li>
        ionosphericPhase: unwrapped ionospheric phase screen. This can be collocated (see <b>Collocate</b> operator)
        with another interferogram with higher spatial resolution.
    </li>
    <li>
        wrappedIonosphericPhase: wrapped ionospheric phase screen.
    </li>
    <li>
        wrappedOriginalPhase: rewrapped phase of original full-bandwidth interferogram.
    </li>
    <li>
        wrappedCorrectedPhase: rewrapped phase of corrected full-bandwidth interferogram.
    </li>
</ul>
<br>

<h4>Parameters<br>
</h4>
<p class="MsoNormal">
<ul>
    <li>
        sigma: standard deviation for defining a Gaussian kenerl for filtering the initial estimation of ionospheric phase screens.
        This value is an integer representing a number of pixels. The Guassian kernel is isotropic, so it is assumed
        that appropriate multilooking has been previously applied to ensure a squared pixel.
    </li>
    <li>
        coherenceThreshold: coherence values below this threshold will be forced to be zero. This is important, as
        the coherence is used to (locally) weight the Gaussian kernel.
    </li>
    <li>
        minCoherence: minimum coherence for masking out values in all output bands. This is mostly for visualization
        purposes.
    </li>
</ul>
<br>

<h4>References</h4>

<ol>
    <li>G. Gomba, A. Parizzi, F. De Zan, M. Eineder and R. Bamler, "Toward Operational Compensation of Ionospheric
        Effects in SAR Interferograms: The Split-Spectrum Method," in IEEE Transactions on Geoscience and Remote
        Sensing, vol. 54, no. 3, pp. 1446-1461, March 2016, doi: 10.1109/TGRS.2015.2481079.
    </li>
</ol>

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